Light emitting diodes (LEDs) are frequently used as light sources in optical communication systems. In such communication systems light pulses act as information carriers for digital signals. However, for example for high bit-rate communication systems the switching on/switching off of the light emitting diodes is critical in that it is not as fast as would be desired.
From an electrical point of view a light emitting diode acts as a normal semiconducting diode but the voltage drop is somewhat higher. The light emitting diode produces an optical power approximately proportional to the driving current. Light emitting diodes have been on-off modulated in a number of different ways.
One known device will now be described. The anode of the diode is connected to a positive supply voltage of for example +5 V. A current generator is arranged between ground and the cathode of the diode although in other known devices the cathode is connected to the negative supply voltage, i.e. ground. The current of the current generator is changing between two fixed levels such as for example 0 mA or somewhat higher and for example 100 mA. The current generator may e.g. be either a bipolar transistor or a MOS transistor. A current generator having an inner parallel resistance is equivalent to an ideal voltage generator having the same serial resistance. Thus another common way of driving a light emitting diode comprises a low resistance voltage generator driving the light emitting diode via a resistor. The voltage generator can e.g. be implemented as a logical inverter. If for example the inverter is a CMOS inverter fed between ground and a positive driving voltage, the light emitting diode is driven by a current charging between 0 mA and a current given by the serial resistance. However, this light emitting diode is shut off completely when it is switched off which is a drawback. Preferably a low current should flow through the diode so that the capacitance of the diode is kept in a charged state which speeds up the switching on of the light emitting diode. In order to provide for a low current through the diode even when it is switched off, an additional resistor R.sub.2 is arranged as can be seen from FIG. 1. In another known device which is shown in FIG. 2 the resistor R.sub.1 of FIG. 1 has been split up in two resistors R.sub.1 ' and R.sub.1 " and a capacitor C has been connected in parallel with the resistor R.sub.1 ". Through the capacitor a peaking of the current through the light emitting diode is achieved. Peaking means that the current at the moment of switching on of the light emitting diode is higher than the final current. The current is also peaked when the light emitting diode is switched off so that the current typically is negative just before reaching the final value.
Generally, peaking is done since the light never follows the current exactly but the light emitting diode acts as a low pass filter from the current to the corresponding light. This means that even if the current is changed momentarily, it takes a certain time such as for example some ns before the light starts to reach its final level. Through peaking the current modulating the light emitting diode, the rise time and the fall time can be speeded up. Often active components such as transistors are used in order to achieve the peaking of the current. However, it is in practice difficult to achieve the negative current that is needed for a fast switching off of the light emitting diode since transistors are unidirectional. The devices as described above suffer a number of drawbacks. For the first, the peaking may either be too low or practically non-existing (FIG. 1) or it is not possible, as often desirable, to keep the supply current to the light emitting diode time invariable. The reason for that is that if the current is for example switched 100 mA during a time interval of for example 0.5 ns there will be a considerable transient on the supply voltage and furthermore radio frequency noise is produced. If then for example sensitive electronic circuits such as for example an optical receiver is arranged closely thereto, it could be disturbed by the radio frequency noise. This situation is even aggravated if the sensitive electronic circuit is located on the same chip since then it is not possible to filter out the noise. Therefore it has become customary to make, for a number of applications, the light emitting diode transmitters differential in order to achieve a constant feeding current. The initially discussed transmitter can easily be made differential but it does not comprise any peaking. The transmitter for which peaking can be provided as illustrated in FIG. 2 on the other hand can not be made differential. Although it would be possible to make a reversely phased identical transmitter, this is not a good solution since it is not at all sufficient to make the transmitter identical with the first one in order to obtain a constant feeding current. Among others the impedance of the light emitting diode is strongly non-linear and therefore the peaking currents upon switching on and switching off will be completely different. Moreover, in practice it is not possible to use an identical transmitter since this would require a further light emitting diode of the same kind which involves too high costs. Thus, even if the light emitting diode transmitter of FIG. 2 provides a comparatively high and well defined peaking which gives fast rise and fall times, it can not be used in applications in which a rapidly varying feeding current can not be accepted.
U.S. Pat. No. 4,818,896 shows an optical transmitter driver with current peaking. The driving circuit generates current pulses suitable for use in driving a light emitting diode at high speeds. The generated pulses contain spikes during turn-on and turn-off in order to quickly charge/discharge the junction and stray capacitances. A separate peaking circuit is provided which comprises a resistor-inductor-capacitor circuit. This peaking circuit is actively connected across the light emitting diode when peaking is required. This means that active components are required among other things in order to activate the peaking function. This arrangement consists of two differential current generators and moreover it has a large current consumption due to the two differential current generators and is too complicated to find a widespread use. It also suffers the drawback of not being flexible.